Gas turbine engines employ compressors and turbines each having arrays of blades and vanes. Each blade or vane includes an airfoil having a suction surface and a pressure surface. During engine operation, a stream of working medium fluid flows over the airfoil surfaces. Under some conditions the airfoil surfaces, especially the suction surface, are susceptible to undesirable fluid separation that compromises the aerodynamic performance of the airfoil. Turbine airfoils that are highly loaded and operate at low Reynolds Number are particularly susceptible to fluid separation. Such highly loaded airfoils are attractive because their use allows an engine designer to reduce airfoil count and thus reduce the weight, cost and complexity of the engine. It is, therefore, desirable to impart separation resistance to such airfoils so that they can be employed effectively.
One known technique for combating separation is to use vortex generator jets (VGJ's). An airfoil designed for VGJ operation includes an internal plenum and a series of spanwisely distributed passages extending from the plenum to the suction surface. During engine operation, pressurized fluid flows into the plenum and through the passages. Each passage discharges a jet of the pressurized fluid (a vortex generator jet) into the working medium fluid flowing over the suction surface. Each jet penetrates through the fluid boundary layer on the suction surface and interacts with the free stream portion of the working medium fluid to create a pair of counterrotating, streamwisely extending vortices in the free stream. The vortices transport higher momentum free stream fluid into the lower momentum boundary layer, thereby counteracting any proclivity for fluid separation. Although this approach is successful, the pressurized fluid used in conventional VGJ arrangements is air extracted from the engine compressor. The air extraction diminishes engine efficiency. Moreover, the supply system required to convey the compressed air to the airfoil plenum introduces mechanical complexity into the engine.
It is, therefore, desirable to devise an airfoil capable of taking advantage of VGJ's without being encumbered by efficiency losses and mechanical complexity.